Sideband generator



P. L. HANCOCK 2,697,220 SIDEBAND GENERATOR Filed May 6, 1953 Q/g R.;

GEN. l

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INVENTOR PETE/ L, HANCOCK ATTORNEY criminating bridge network Patented Dec. 14, 19:14

SIDEBAN D GENERATGR Peter L. Hancock, Rochelle Park, N. J., assignor to Inter- `national 'ielepiione and telegraph Corporation, a corporation of Maryland Application May 6, 1953, Serial No. 353,419 7 Claims. (Cl. 343-107) This invention relates to sideband generators and more particularly to mechanically driven sideband generators.

ln general, sideband generators are intended to provide the necessary radio rrequency signals for either the course or path racilities of an instrument landing system. Heretot'ore, reactance type sideband modulators have been used which caused the load impedance to theV radio frequency generator to vary. Since the radio frequency generator had to be designed to function with a varying load impedance it could not be operated at a high order of efficiency. When two sideband rrequencies had to be generated, as is required for most instrument landing systems, an isolation network between the sideband trequency generators had to be provided to minimize the excessive cross modulation and even with such networks there always remained a residual amount of Other di'fliculties have arisen in the the` transmission lines carrying radio frequency power which caused an increased stability requirement on the generator.

One of the objects of this invention, therefore, is to provide a sideband frequency generator system which offers a substantially constant load impedance to the radio frequency generator.

Another object of this invention is to provide a sideband frequency generator system to generate a plurality of sideband signals of different frequencies without the occurrence of cross modulation or the necessity of an adjustment in an isolation network to minimize such cross modulation.

t A further object of this invention is to provide a sideband generator system having a greater stability in the modulated waveform through the independence of the modulator parameters on the source of impedance. A feature of this invention is the use of a radio frequency generator whose output energyA is coupled to two identical phasing section bridge networks each having equal distances between their junctions with 180 added to one leg of each bridge. Motor driven balanced capacitors are provided to vary the admittances terminating the side arms of the phasing section bridge networks in atreciprocal manner to cause the output energy to be a sinusoidally varying sideband modulated radio frequency signal which is coupled to the input legs of a phase diswhere the radio frequency carrier energy is canceled coupling pure sideband energy to two equal loads of equal impedance.

` The above-mentioned and other features and objects ofthis invention will become more apparent by reference to the following description taken in conjunction with j `the accompanying drawings, in which:

`Fig. 1 is a schematic illustration in block form of one embodiment of the sideband generator of this invention; Fig. 2 is a vector diagram used in explaning the operaton of the sideband generator system of Fig. l; and

Fig. 3 is a block diagram of an instrument landing system using the sideband generator of this invention. j Referring to Fig. 1 a sideband generator in accordance with the principles of this invention is shown comprising a radio frequency generator 1 coupled to the input junctions of two identical phasing section bridge networks Zand 3. The side arms of the phasing section bridge networks 2 and 3 are coupled together through two motor `driven balanced capacitors 4 and 5. The sinusoidally 0 capacity may be obtained.

capacitors provide the correct varying sideband modulated radio frequency signals from the output terminal oi eacn oi me bridge networks Z and J are coupled to me siue arms or a phase discriminating bridge o whose output is coupled to two loaus 7 and o oi equal impedance.

nach phasing section bridge network 2 and 3 is so constructed that there is equal etectrlcal lengths degrees between eacn ot' its junctions witn au additional distance equal to izU in electrical length added to one arm in caen bridge t0 provide the equivalent Oi me usual transition. lne raulo lrequency energy trom generator i is coupled to the input terminals Atl and .att of bridges 2 and J respectively. 'lne transler ot energy between the opposite junctions Ztl, 2D and 3a, .sa ot' bridges 2 and s respectively is proportional to the dltference or' the admittances terminating lne side arm junctions zc, 2a and 3c, sa. when tne admittances are susceptances only, that is, when their renection coem'- cients are equal to unity and or sucn an angie that at their respective junctions me vector sum is equal to zero, the transrer or energy between the input terminals '2a and 3a and the output terminals 2o and 3b is complete. flhe phase or the resulting voltage output b2b and nab at me load junctions n anti 30 with respect to tne input voltages liza and lisa at junctions a and 3u is at any instant or a moduiatiiig cycle the angle determined by the vector sum or voltages coupled to junctions La and 3b trom the side arm junctions 2c, 2d, and 3c, 3d. ln order to vary the admittances terminating the side arm junctions ot bridges 2 and to vary the transter or energy between the input and output terminals, corresponding side arm junctions 2c, 3c, and 2d, 3d of the phasing section bridge networks 2 and 3 are coupled togemer. Balanced capacitor 4 having two capacitor sections 4a and 4b is coupled between the side arm junctions 2c and 3c. The electrical length trom terminal 2c to capacitor section 4b is 90 greater than the electrial length between the terminal 3c and capacitor section 4a. ln a similar manner balanced capacitor 5 having two capacitor sections 5a and 5b is coupled between side arm junctions 2d and 3d, the electrical length from terminal 3d to capacitor section 5a being 90 greater than the length between terminal 2d and capacitor section 5b. The balanced capacitors 4 and 5 are driven by a motor 9 through mechanical linkage 10. The motor driven balanced shunting susceptances at the side arm junctions of each of the phasing section bridge networks 2 and 3 while the correct sign for the susceptances is provided by the ditference in the telectrical distance from the side arm junctions of each bridge network to the capacitor sections.

Each capacitor section of balanced capacitors 4 and 5 may have a multi-bladed rotor. The desired modulating frequency is then determined by the number of rotor blades times the speed of the motor in seconds. to the location of motor driven balanced capacitors 4 and 5 the admittances terminating the diagonally opposite side arm junctions of each bridge 2 and 3 varies reciprocally in a sinusoidal manner. It is well-known to those skilled in the art from the known relation of power transfer through a hybrid ring or bridge network that the impedance represented by each of the phasing section bridge networks 2 and 3 is constant for a unity reflection coefficient at the two side arm junctions 2c, 2d and 3c, 3d with the vector sum at the output terminals 2b and 3b equal to the generator voltage if the susceptances are equal and of conjugate phase. Hence the total input impedance to generator 1 remains a constant.

Of course it will be understood that the motor driven balanced capacitors used to vary the admittances terminating the side arm junctions of each bridge may be replaced by any equivalent circuit element such as balanced reactance tubes.

By the proper choice of line length 1 between the capacitors 4 and 5 and the bridge networks 2 and 3, and of the characteristic imbedance of the side arms to the capacitors 4 and 5, and in the value of the minimum capacity of each section of balanced capacitors 4 and 5, a practical capacitor with a practical incremental In order to insure the production of the required incremental impedance at the putrof 3 capacitors 4 and 5 shunting inductances 11 and 12 are provided which permits a practical and simple means for adjusting the resultant incremental impedance over a band of.operatingjrequenciest Y j ,y varying amplitude modulatedradio frequency voltages are coupled to Aterminals 6c `and 6d .of phase discrirn inating bridge network 6. Bridge network (6, is si ilar in construction to networks 2 and 3, that is, lthe;l isjequal distance between each of its junctions with Fan eXtra 180 in electrical distance added to gone arm.4 In accordance with well-known bridge network theory the radio frequency carrier energy cancels and pure sideband energy is coupled from the output terminals 6a and 6b to loads 7 and 8.

Referring `to the vector diagram vof Fig. 2', if for an instantaneous value in phase difference between Ezd and Esa, the two input voltages are in phase as shown by vectors AO and OB, the voltage 'output from terminal 6b of bridge network 6 will be zero since the 'input voltages cancel: due to the 'tr n'sitio'n arm between terminals 6d and `6b being 180 greater in length than the bridge yarmbetween terminals 6c and 6b. However, under the same conditions 'of inpunphase relation the voltage Aoutput from terminal '6a will be equal to the 4sum of the vectors 'AO #and OB since the input voltages Een and Esa is made 'to 'advance '90 `in Vector OD and input voltage 'Esa is lretarded vby 90 in phase, as 'represented by vector OC vthe resultant VO'ltv age 'output from terminal 'f 6b lis thefsum 4of Tthe input voltages Eea and Erd diie to the 'transition afrm of network 6 while 'the `resultant voltage output from terminal 6a "becomes 'z'eijo s'ice l`the ltwo input voltages cancel. If

'we assume that `the input voltage En iis retarded 90 in phase fromits po'sition'O the vector Amayl'be represented byoo an-'dif 'ihefphase 1er-the input vonage En 'is advanced`by-90" it is represented'by vector OD and the v'resultant voltage' at output terriiiiia1`6b is -again equal to the sum ofthe input voltages, b` Vt lit isnow 180 'outof phase witnrespect to vthecondition describedabove. For 'any phase change intermediate "of the conditions, described, 'the voltage `output-from terminal 6bris equal lto Vthe vectorial'dilferen'c'eof'the input voltages 'ErdV and Esa andthe output voltage `from terminal 6a is equal -to the vectorial addition of the input voltages kEzd and Ess. bviously if the;velocit`y and the Vchange in phase of ythe two input voltages Eze "and Esa is constant, their respective phases can berepres'ented byA two contra- `rotati`ng vectors as"show'n"by vectors Eni and Bad, then ,sinusoidal varying sideband signals are coupled from output terminals`6a andf 6b 'to ,loads 7'and18.

Referring. to Fig. `3 of the drawing 'a's' hematic illustration in block lform of 'an"instru`rnent landing transjmission-systern is shown wherein thegeneration of two sideband signals of di'fferentjfrequency is accomplished without the 'occurrence of f cross `modulation :or 'the necessity 'of an '-adjustmentandlisolation znetwork to rriinirnize cross modulation. 'A' radio frequency generator 14 supplies carrier `frequency energy to vtwo sideband gen erator circuits and 16. Each fsideband generator' circuit 15 and 16 is similar to the circuit Wshown in Fig. 1; "however, `the modulation frequency in 'each circuit is different, -for example,"'one-jsideband generator 1S -generates'90 cycles per "second while the "other sideband generator 16 generates' 150 cycles per second.`v The out- 'each Vsideban'd generator is coupled to the instrument 'landirigsyste'm antenna array `1,7. "The output of generator '14 is coupled through'power divider 18 to Pthe "antenna array 17 to-providecarrier frequency energy. Power divider 18 provides an adjustment for the per- `phase as represented by f centage modulation, which can, if necessary, also lbe xed at the designed center v alue by the power division between the carrier and sideband signals. Since in all transmission lines carrying radio frequency energy no standing waves exist, spurious radiation is minimized and the efciency of the instrument landing system is in creased.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set forth in the objects thereof and in the accompanying claims.

I claimi 1. A sidebandfrequency generator comprising a iirst and second bridge network each having one leg thereof arranged to include an equivalent of a one-half wavelength greater length as compared to thelength of each sinusoidal of the 'other three legs, means to vary in a 'and complementaryrmaniier the admittances terminating two diagonally opposite junctions -of each bridge, `a source of radio frequency energy, meansto apply said radio frequency energy to one other junction 'of leach bridge, a third bridge having one leg thereof arranged to include an equivalent of a one-half wavelength greater length 'as compared `t`o the length of each 'of 'the other three legs, means to couple the two diagonally opposite junctions of said third bridge to the output from the junction opposite said other junctions in each of said ,rs't and second 4bridges 'and means for connectingtwo loads to the other two diagonal junctions 'of said third bridge. Y h

2. A Asideba'nd frequency generator according to claim 1 wherein said 'admittances terminating the diagonally 'opposite junctions of Veach `of 'said iirst and second bridges are pure susceptances having a vreiiection coeicientequal to unity and Yof 'such an angle ythat at their respective junctions the vector Vvsum is zero. y l

3. A sideband frequency generator according to lclaim 1 wherein said means to vary in *a sinusoidal and coin'- plementary manner the admittances terminatingtwo `Adi- 'agona'lly opposite junctins vof each 'offsaid first and slec- ,ond bridges includes a lmotor, `a first'and second variable balanced'capacitor driven `by said motor, each capacitor having a first and second capacitivesection, a ifirst pair of circuit means coupling the rst 'sectiorrof said rstand 'second balanced l`capacitors lrespectively to "diagonally opposite junctions `of said rst bridge, -a second vpair o circuit means coupling lthe second s'ectionjof said afirst and second'balanced'capacitors respectively 'to diagonally Vopposite junctions of said second bridge, Vsaid 4instand wavelength j greater 'length than ithe length'of the-'other f circuit thereof.

'4. Asideband frequency generatoracc'ordingo claim 3 which further includes a variable` shunting `induc'tance connected in parallel across'eachof said :motor-driven balanced capacitors. y

5. A sideband frequency 'generator ycomprising*a"first and second bridge network each having'one ;leg zther'eof arranged to includeanequivalent'bf aTone-'half'wavlelength greater length as4 compared to jthelengthof each of the other three legs, meanstovary"in-a'sinusoidaland complementary manner the admittances which'lrave la reflection 4'coeiiicient equal 'to v.unity terminating Ltwo diagonally opposite junctions of each `of l'said iirst 'and second bridges including 'a vm'otor, -a "rst and second variable balanced capacitorV driven "by said motor, each capacitor having a first 'and "second ca'fyeffrtve section, a rstpair of circuit 'means 'coupling'therst section of said first and second'balanced capacitors "respectivelyto diagonally opposite june fons lof said'irstbri'dgz, a ond pair of circuit means couplingth'e second sectiono said first and second balanced capacitors respectively t'oliagonallyvopposite" junctions .of fsaid second bridgegsaid `first and second circuitlrneansbeingjairanged to include diagonally opposite junctions of said third bridge to the output from the junction opposite said other junctions in second bridges and means for connecting two loads tothe other two diagonal junctions of said third bridge.

6. An instrument landing system comprising a source of radio frequency energy, means to couple a first and second sideband generator to said source of energy, each of said generators including a first and second bridge network each having one leg thereof arranged to include an equivalent of a one-half wavelength greater length as compared to the length of each of the other three legs, means to vary in a sinusoidal and complementary manner the admittances terminating two diagonally opposite of radio frequency energy to said antenna system.

7. A system according to claim 6 wherein said means for coupling said source of radio frequency energy to said antenna array further includes means to control the amount of energy coupled from said source of energy to said antenna array.

References Cited in the file of this patent UNITED STATES PATENTS Number NameV Date 2,414,431 Alford et al. Ian. 2l, 1947 2,429,634 Lundberg Oct. 28, 1947 

